The present invention relates to a method and apparatus for inspecting conductive surfaces for small near surface fatigue cracks or flaws in an in-service or production line environment. To accommodate stringent inspection requirements, e.g. government safety specification tolerances on aircraft engines, the inspection system must be capable of detecting and characterizing flaws heretofore undetectable by conventional techniques. Image processing techniques have long been employed in visual, X-ray and ultrasound inspection systems but are not commonly applied to eddy current nondestructive testing (NDT).
One method of eddy current NDT uses a probe comprising two separate coils operating in reflectance mode. If the eddy current probe contains two separate coils,, drive coil and sense coil, it is said to be operating in reflectance mode. If the probe drive coil surrounds two sense coils connected in series and both sense coils are on the conductor surface it is said to be operating in differential mode. If one sense coil is on the conductor surface and the other sense coil is on a fixed reference (conductor or air) the probe is operating as an absolute, probe. The advantage of using probes operating in reflectance mode is that drive and sense signals on respective coils are easily separable, whereas in differential probes they are not due to a bridge circuit mode of detection. Operation of the reflectance mode probe involves measuring the voltage difference at a sense coil when it is excited by a frequency driven alternating current flowing in a nearby drive coil. The potential difference of the sense coil changes near the surface of a conductor as currents are induced to flow in the conductor in response to the magnetic flux generated by alternating currents flowing through the drive coil. As the drive coil is brought near the surface of a conductor, current will be induced in the conductor near the surface and will remain under the probe as it is moved across the surface. The voltage measured will be approximately constant until a near surface fatigue crack or flaw is encountered. At that time, the induced current flow is disrupted or deflected and the measured impedance changes. This disruption is measured as a change in sense coil voltage indicative of the flaw signal measured by a reflectance mode eddy current probe.
Typical eddy current probes employed for NDT in the industrial inspection of conductive parts use inductive coil elements wound around ferrite cores to intensify induced magnetic flux. The coil winding radius onto the ferrite core presents a limit on the flaw size that can be detected. Inspection is usually accomplished by having an individual split core differential probe traverse the surface of the conductor in a repeat pattern to adequately scan the surface area of the conductive part under inspection. Conventional flaw detection is accomplished by monitoring the sense coil response signal of a single scanning probe to detect a voltage disruption threshold over noise for that probe. The smaller the flaw, the harder it becomes to distinguish flaw signal from background noise. Prior art monitoring for occurrences of such single threshold phenomenon using single probe inspection scanning is time and labor intensive and not conducive to image processing of automatically acquired measurement signals. Spatially correlated measurements can be obtained from this type of single probe traverse scanning wherein time histories along the scan path are retained and used for imaging. Flaw detection often involves looking for the recurrence of a difficult to distinguish threshold flaw signal that might otherwise be considered as noise. The use of imaging techniques as applied to eddy current Non-Destructive Testing (NDT) was first suggested by D. Copley, Review of Progress in Quantitative Nondestructive Evaluation, Vol. 2B, Plenum Press New York, 1983, p. 1527. Copley introduced imaging based on asynchronously acquiring one dimensional signals for two dimensional formatting without any refinement. Imaging in this manner is accomplished by indexing signal intensity as measured at predetermined positions along the scan path to an ordered two dimensional array to provide crude spatial correlation of asynchronous measurements. The problem with this imaging approach is that it is time and labor intensive. Furthermore, imaging is spatially blurred due to the much larger relative size of the probe compared to the flaw. This masks refinement efforts, making it difficult to interpret underlying defects.